Here are a few suggestions. Go late (9 or later), otherwise, you may be the only people in the place:

For what many consider to be the best food in town:
-Bakea in Lomas de Chapultepec (French-Basque)

If you are more about the romance/atmosphere than the food (although the food can be quite good)
-Lago del Chapultepec in the park
-Bellini (on top of the World Trade Center)
-LeCirque in the Camino Real (a branch of LeCirque 2000 in New York)

Good food, more of a local scene, less pricey than the above:
DO in Polanco
Naos on Palmas

Also, while you are here, take a tour through the Condesa. Start with a drink in the Condesa DF hotel, then walk down Tamulipas and Michoacan streets and check out the bars and restaurants along the way that catch your eye. It could be the most romantic evening of your stay.

Have fun.